As known, a solid-state laser (SSL) extracts coherent light from an inverted population of neodymium, ytterbium, or other suitable ions doped into the SSL gain medium of an optical resonator. This population inversion is created by optically exciting the dopant ions by pumping them with optical radiation at wavelengths shorter than the laser wavelength. The pumping raises the laser atoms to an upper energy level, thereby increasing the laser power. The pumping, however, also generates heat that creates transverse temperature gradients within the SSL gain medium. These temperature gradients can distort the optical phase front of the laser and degrade beam quality, thereby limiting the resonator's ability to produce near diffraction-limited beam quality (BQ).
There is, however, a class of SSL gain medium known as “active mirror amplifier” (AMA) which exhibit lower transverse temperature gradient characteristics and are capable of providing good laser BQ in high average power (HAP) SSL applications (in excess of 200 watts). These AMA gain mediums have several known configurations. One such type is disclosed in U.S. Pat. No. 6,339,605 to Jan Vetrovec, entitled Active Mirror Amplifier System and Method for a High-Average Power Laser System, which is incorporated herein by reference. It uses a large-aperture solid-state laser gain medium disk which is about 2.5 millimeter (mm) thick, with a selectable diameter of from 5 to 15 centimeter (cm) diameter, and which is mounted on a rigid substrate, or optical bench. The substrate is cooled by a gas or liquid medium that circulates in microchannels embedded in the interior of the substrate, which in turn keeps the gain medium disk cool.
An alternative AMA is disclosed in U.S. Pat. No. 6,625,193 to Jan Vetrovec, entitled Side-Pumped Active Mirror Solid-State Laser for High-Average Power, which is also incorporated herein by reference. There a large aperture gain medium disk is optically pumped by radiation injected into the peripheral edge of the disk. Side-pumping takes advantage of the long absorption path (approximately the same dimension as the disk diameter), which permits doping the disk with a reduced concentration of lasant ions, and provides a corresponding reduction in required pump radiation intensity, and heat. A further alternative type AMA is shown in U.S. Pat. No. 6,810,060 to Jan Vetrovec, entitled High-Average Power Active Mirror Solid-State Laser with Multiple Subapertures, which is also incorporated herein by reference. Once again the gain medium disk is attached to a cooled, rigid substrate as in the '605 patent, but here the large optical aperture of the disk is filled with multiple AMA subapertures.
Each of these different type AMA gain medium allow for generation of a near diffraction limited laser output from the AMA at very high average power. A HAP SSL optical resonator can then be achieved by combining several of these AMA gain medium modules within the resonator cavity. In the prior art this is achieved by optical resonators which have a linear or a circular optical resonator configuration. Each of these configurations, however, require relatively large optical cavities to ensure adequate beam quality (BQ). In linear resonators the cumulative beam propagation path length required for the laser to travel between AMA modules adds significantly to the overall length and weight of the resonator. Alternatively, circular optical resonators present a high angle of incidence to the laser radiation at the gain medium optical surface, making water cooling of the AMA more complex. This too adds size and weight to the resonator structure. The larger size and weight of the linear and circular resonator structures also cause them to lose optical bench stiffness, further contributing to resonator instability.
There is therefore a need for an optical resonator configuration which overcomes the size and weight disadvantages of the prior art linear and circular AMA optical resonators, and which more readily provides the ability to scale the resonator to higher output power with greater numbers of AMA modules than is possible with prior art resonators.